The invention relates to a spindle drive for longitudinally adjusting a motor vehicle seat.
A spindle drive illustrated in FIG. 1 is known from DE 10 2006 058 361 A1, which comprises a spindle 1, which engages with an adjusting mechanism 2, which is connected to the adjustment rail of the longitudinal seat adjustment for the vehicle seat, for instance an upper rail mounted sliding in a lower rail connected to the motor vehicle floor. A fixing bracket 5, preferably out of metal and in particular out of steel, serves for mounting the adjusting mechanism, wherein said fixing bracket comprises a base 50 and two lateral arms 51, 52, on which in each case through-openings 53, 54 for the spindle 1 are arranged. A fixing flange 55, 56 with a fixing position in form of a fixing opening 57, 58 projects in each case from the two lateral arms 51, 52 of the fixing bracket 5, between which and the base 50 the adjusting mechanism 2 is arranged, wherein via said openings the fixing bracket 5 is mounted to the adjustment rail.
The adjusting mechanism 2 comprises a transmission housing 6, in which a spindle nut 3 and a driving worm 4 are arranged and mounted. The spindle nut 3 contains an internal toothing being engaged with the external thread 10 of the spindle 1 as well as an external toothing, which meshes with the driving worm 4. When operating the spindle drive, the spindle nut 3 is set into a rotational movement via the driving worm 4 being connected to a driving device, wherein said rotational movement is being transferred due to the engagement of the internal toothing of the spindle nut 3 into the external toothing 10 of the spindle 1 into a longitudinal movement of the adjusting mechanism 2 relative to the spindle 1 so that the adjusting mechanism 2 is adjusted relative to the spindle 1 being connected to the vehicle stationary and torque-proofed, and adjusts the adjustment rail of the longitudinal adjustment of the motor vehicle seat connected to the adjusting mechanism 2 and thus adjusts the motor vehicle seat in longitudinal direction of the spindle 1 relative to the motor vehicle floor.
In order to guarantee a secure and tight fixture of the upper rail, the adjusting mechanism 2 is dimensioned such that it can capture and support the forces introduced from the adjustment rail during a normal adjusting movement of the motor vehicle seat. In case of an impact, in particular during a frontal or rear end impact of the motor vehicle, it has however to be ensured that the adjusting mechanism 2 does not slides through in longitudinal direction of the spindle 1, since this would lead to an undesired acceleration of the motor vehicle seat connected to the adjustment rail of the longitudinal seat adjustment and thus to an increased risk of injury for a motor vehicle occupant residing on the motor vehicle seat.
In order to allow for a simple, easy and cost-efficient construction of the adjusting mechanism 2, the transmission housing 6 and the spindle nut 3 are preferably formed of plastic, wherein the spindle nut made of plastic also contributes with its plastic toothings to a minimization of noise during an adjusting movement.
For increasing the strength and stability of the adjusting mechanism 2, the adjusting mechanism 2 is encompassed by the fixing bracket 5 made of metal and is connected via the fixing bracket 5 to the adjustment rail of the longitudinal feet adjustment, so that in case of an impact the fixing bracket 5 is deformed by the spindle nut 3 and the transmission housing 6, until depending on the occurrence of a frontal or rear end impact the respective through-opening 53, 54 of the fixing bracket 5 grips into the thread of the external thread 10 of the spindle 1 and through this secures the motor vehicle seat connected to the adjusting mechanism 2 via the adjustment rail of the longitudinal seat adjustment in case of an impact against an undesired longitudinal adjustment.
Since the requirements to the impact stability of the spindle drive also relate to extreme climate conditions with temperatures in a range of −30° C. to +80° C., which could lead to an increased flow or embrittlement of the plastic in case of a spindle nut 3 consisting of plastics due to weight and production reasons, the danger exist that the forces acting in case of an impact are not longer sufficiently transferred from the spindle nut 3 via the transmission housing 6 to the fixing bracket 5 in order to deform said bracket for gripping in the spindle 1 so that the danger of a non-locking of a longitudinal seat adjustment of a motor vehicle seat in case of an impact exist at least at extreme temperatures.